Performance of conventional solar cell panels depends upon the size, shape, flexibility, and packing efficiency of the individual solar cells that are assembled to make up the panels. These relatively large (1-9 in.sup.2) size of the solar cells and arrays affect the performance of the cells and complicate their mounting on small, irregularly shaped, or nonplanar body surfaces. For optimal array performance, every cell in an array should be receiving substantially the same illumination. The efficiency of the cells falls dramatically as the angle of incident radiation deviates from being normal (90.degree.) to the surface. Yet, to cover the exterior of a satellite requires that the cells curve around in discrete segments or facets being some multiple of the characteristic dimension of an individual cell. To generate the desired spacecraft power, multiple cells must be interconnected. A cell's voltage is a function of the material while its current relates to the efficiency of illumination. If the curvature of the array (or offset from normality with the incident radiation) exceeds about 15.degree. of arc around the exterior surface of the satellite, the differences in illumination will be so significant. The array will generate for all the interconnected cells at the performance of the weakest cell. The total power generated will be a fraction of what could be generated if the array and subcircuit designs achieved substantially equal illumination for all the cells in a given zone of intensity.
Amorphous silicon solar panels have been made on flexible substrates, and could be made into irregular shapes; however, amorphous silicon degrades in sunlight. These flexible cells could conform with the spacecraft's surface, but the low conversion efficiency (8-10%) and the degradation make them undesirable.
Power is a function of the cell's conversion efficiency, its characteristic voltage, its generated current, and area. To achieve the desired subcircuit voltage for a useful space power subsystem, a predetermined number of solar cells are connected electrically in series. Each cell contributes an increment to the array voltage. The voltage each cell produces depends on the material from which the cell is made and the operating conditions, primarily the temperature. For example, silicon cells generate about 0.5 volts while gallium arsenide (GaAs) cells generate about 0.9 volts at room temperature. A subcircuit designed to produce 36 volts (a common spacecraft power system voltage) requires 72 silicon cells or 45 GaAs cells. The total area is 72 times the individual silicon cell area (or 45 times for GaAs cells) divided by the packing factor (usually approximately 0.9). Therefore, a subcircuit made up of common 2.times.2 cm (i.e., 1 in.sup.2) silicon cells would have a total area of about 320 cm.sup.2 (about 200 cm.sup.2 for the GaAs cells). Areas of substantially equal illumination this large are often unavailable on the exterior surfaces of satellites, so the satellites need to carry a complicated folding solar power panel and a tracking system to point the panel at the sun. Smaller surface areas on the satellite are essentially wasted because covering the smaller areas with conventional solar cells does not produce power at the desired 36 volts or the illumination of interconnected yet dispersed cells may be so different that the array generates virtually no power, corresponding to 72 or 45 times the weakest cell.
All cells within a given subcircuit in an array must be identical in area, perform at the same efficiency, and receive the same solar fluence (i.e., be illuminated equally) to generate the same amount of electrical current. If the current from cells in an array is not matched, the output of the subcircuit will be reduced. Either the current in the subcircuit will be reduced to that of the weakest cell, or the weak cell will be forced to operate in reverse causing resistive heating and a subsequent reduction in total subcircuit voltage.
Nonuniform illumination of the cells can result from a shadow cast across any cell within a subcircuit or from curvature of the array. Nonuniform illumination causes some cells to operate at reduced current and voltage. When large area subcircuits experience nonuniform illumination, the area effectively removed from power production can be much greater than the actual area that is in the shadow. The current from the entire subcircuit is reduced to the current produced by the least illuminated (i.e. the weakest) cell.
A need exists for low-cost, self-contained photovoltaic power subsystems for use on microsatellites that take advantage of small, irregularly shaped, or nonplanar surfaces to capture solar energy. Such systems should be adaptable to non-rectangular and nonplanar surfaces and be sized so that, when shadows fall on the cells, only the actual shadowed area is removed from power production. In addition, the system should lend itself to processing steps that are reliable and economic. Microelectronic processing techniques, like photolithography, allow the manufacture of small cells of equal area of regular or irregular shape. Small cells allow the construction of microarrays. For example, if a 2.times.2 cm silicon solar cell were partitioned into 72 active areas connected in series, the 2.times.2 area would produce 36 volts yet the array would be tiny with respect to conventional designs. Each cell would be about 0.1.times.0.1 inch (0.25.times.0.25 cm). The present invention involves design and fabrication of microarrays using these tiny cells. Microarrays occupy small areas and enable smaller spacecraft with reduced mass and with optimally tailored power. Microarrays can be made with any planar solar cell phototransducer, such as crystalline Si, GaAs, GaInP/GaAs, or .alpha.-Si. Because the cells in a microarray occupy a smaller total area, it is easier to arrange the cells on the satellite's exterior so that they are equally illuminated.